1. Field of the Invention
This invention pertains to the field of producing ammonia. More specifically, the present invention relates to reducing the costs of producing a unit amount of ammonia, particularly the energy costs, by the utilization of an enhanced boiling surface in the refrigeration zone of the process.
2. Discussion of Related Art
Ammonia is produced in large quantities throughout the world. Most of the current production processes in use involve the reaction of hydrogen and nitrogen at high pressure. Indeed, the synthesis of ammonia from hydrogen and nitrogen was the first chemical reaction to be carried out under high pressure on an industrial scale.
The reaction of nitrogen and hydrogen to form ammonia takes place in the gas phase and proceeds with a decrease in volume. Accordingly, the equilibrium conversion or yield is favored by high pressure and/or low temperature, as one skilled in the art would readily appreciate. Typically, the pressure in the ammonia synthesis process is in the range of from about 1,000 to 2,200 psia, or higher.
Generally, an ammonia synthesis gas mixture comprising a stoichiometric amount of hydrogen and nitrogen (3:1) is first compressed in a synthesis gas compressor to the required high pressure and then passed to a converter reactor. In the converter, with the aid of an appropriate catalyst, the synthesis gas is reacted to form gaseous ammonia, typically in an amount of from about 9 to 11 mole percent ammonia. The ammonia is generally recovered from the reacted synthesis gas mixture by a series of ammonia refrigerated chillers connected in series, which condense the gaseous ammonia product between its dewpoint (about 75.degree. F. for a stream containing about 9.5 mole % ammonia at 2100 psia) and about -10.degree. F., using ammonia refrigeration as low as -28.degree. F.
The ammonia refrigeration is provided by an ammonia refrigeration multistage compressor. The total power expended by this compressor is the sum of the power consumed by each stage of the compressor, respectively, which is directly proportional to the flow through each stage and is exponentially proportional to the pressure ratio across each stage.
Following condensation, the separated liquid ammonia is purged of any dissolved gases, and stored either as a liquid, or piped as a gas to the end user.
At the above noted pressure levels, the ammonia in the reacted synthesis gas stream begins to condense at about 75.degree. F., and at -10.degree. F. about 80% of all of the ammonia present in the stream will have condensed. If the reacted synthesis gas is further cooled to -25.degree. F. (about the limit for a refrigeration system based on ammonia as the refrigerant), then about 87% of the ammonia contained in the synthesis gas will have condensed. For a synthesis gas containing about 9.5 mole % of ammonia, a recovery of 87% represents the upper practical value of recovery based on the given refrigeration system.
Some commercial processes recover yet addtional ammonia by vaporizing ammonia refrigerant under vacuum to achieve still lower temperatures. Typically, however, the lowest refrigeration level is limited to about -28.degree. F., the normal boiling point of pure ammonia at atmospheric pressure.
As in any production process, there is a desire to reduce the costs associated with the production of the ammonia while at the same time, seek a greater output of ammonia product. While this is an admirable objective, it is not always easily obtainable.
As is apparent from the above, there are essentially three main constituents in the ammonia synthesis process, namely, the converter reactor, the synthesis gas and refrigeration compressors, and the ammonia refrigeration chillers.
In general, the common approach to improving the ammonia synthesis process is to attempt to upgrade the converter. In this manner, the reaction produces a reacted synthesis gas having a larger concentration of ammonia, i.e., a greater reaction yield is obtained. An increase in ammonia concentration, however, as understood by those skilled in the art, corresponds to a similar increase in the loading and power consumption of the refrigeration system which usually requires the addition of a larger driver for the compressor.
Since the multistage compressor is a highly expensive piece of machinery involving not only a high initial capital cost but also high operating costs as well, the expected need to increase overall power consumption to accomodate such an added increase in ammonia production may not always be desirable.
As to the compressors, they are usually sized to accomodate particular design considerations for the specific process including flows, temperatures, pressures, etc., and will usually operate at a constant required amount of power consumption for the given process. The power consumption of a compressor, such as the multistage refrigeration compressor, is directly related to the flow across each stage of the compressor as well as the pressure ratio across each stage, which are both generally a function of the amount of ammonia being produced. Thus, the more ammonia being produced, it is generally expected that the more energy will be expended by the compressors.
With respect to the refrigeration chillers, which are typically shell and tube type heat exchangers, because of the high operating pressures involved in the ammonia synthesis process, these chillers are generally expensive to construct and require high pressure closures on their heads and channels. One common type of chiller uses all welded construction with no gasketed channel covers to prevent leakage.
Due to the high pressures, it is also desirable to keep the chillers relatively small to minimize metal thickness on the tubesheets and channel barrels. It is also desirable to avoid splitting a particular stage of chilling to more than one shell so as to avoid phase separation and temperature maldistribution on the synthesis gas side which adversely affects ammonia recovery. Moreover, leakage and piping costs are minimized if only one shell is used for each of the chilling stages.
A final requirement in the design of the synthesis gas chillers is to maintain sufficient temperature difference between the condensing ammonia and the boiling refrigerant ammonia. Typically, these chillers have cold end temperature approaches of about 15.degree. to 20.degree. F. The phrase "cold end temperature approach" is meant to specify the difference in temperature between the cooled reacted synthesis gas leaving the chiller and the ammonia refrigerant vaporizing in the chiller.
While lowering the cold end temperature approach would permit more ammonia to be recovered, it would also simultaneously and undesirably require larger, more expensive chillers or, alternatively, more than one chiller in parallel per stage, in order to do so. The combination of greatly accelerating costs with surface area and the adverse effects of flow maldistribution in parallel units, are factors which have lead away from design of such chillers with cold end temperature approaches less than 15.degree. to 20.degree. F. in ammonia plants commonly built in the 1960s and 1970s.
While an attempt has been made to improve the efficiency of the ammonia production in process by using a plate-fin or plate frame type of exchanger in the chillers in order to increase the surface area per unit volume of exchanger, such as is disclosed in U.S. Pat. No. 4,689,208, such an approach has the primary disadvantage of being limited in operating pressure to relatively lower pressures which, as noted above, does not favor the synthesis reaciton.
A need clearly exists therefore, to improve the ammonia production process, particulary to decrease the amount of energy expended to produce a unit amount of ammonia.